Steel for elevated temperature service



Patented Dec. 13, 1949 STEEL FOR ELEVATED TEMPERA- TURE SERVICE Rufus E. Zimmerman, Short Hills, N. J., as-

signor to United States Steel Corporation of Delaware, a corporation of Delaware No Drawing. Application May 21, 1948, Serial No. 28,517

1 Claim.

This invention relates to improvements in the low-carbon, molybdenum type of steels frequently used for boiler and superheater tubes, high pressure steam pipe and the like, and which are subjected to internal pressures at temperatures ranging from 850 to 1100 F. during service.

Steels designed for such service must have high resistance to creep at the foregoing temperatures over extended periods of time, must be capable of being readily forgeable and form'able and, at the same time, should have good tensile properties at room temperatures. While the foregoing properties can be more or less readily achieved by resorting to high alloy steels, it is commercially and economically essential that a minimum of alloying additions be resorted to due to the cost factor, since such steels are so widely used.

For certain purposes. it is desirable to make the steel by the so-called fine grain deoxidation practice involvin the use of 2 to 3 lbs. of aluminum per ton of steel, whereby a residual aluminum content of at least .02% is obtained in the steel. This however, has the deleterious efiect of causing the steel to graphitize rather rapidly when used in the foregoing temperature range, and seriously weaken the steel due to the graphitization producing planes of weakness. It has, heretofore. been proposed to overcome such graphitization by adding .15 to less than 1% chromium to such steels. However, it has been found that chromium in these amounts is insuflicient in all cases to prevent complete graphitization when used in the foregoing temperature range. Although additional chromium will produce the desired result, the amounts needed, i. e., in excess of 1%, are economically undesirable due to the greatly increased cost and meallurgically undesirable due to the chromium causing air hardening thereof.

It is accordingly an object of the present invention to provide a low-carbon, molybdenum steel which has ood properties for high temperature service including resistance to graphitization at relatively low cost.

It is another object of my invention to provide a low-carbon, medium molybdenum steel which combines the qualities necessary for high temperature service with good formability and room temperature tensile properties.

In accordance with the teachings of my invention, aluminum killed low-carbon, molybdenum steel containing .05 to 20% carbon, .40 to 1.00% manganese, .20 to .30% silicon, .50 to 1.50% molybdenum, .05% maximum phosphorus, 04% maximum sulphur, at least .02% residual aluminum, with the balance substantially iron, except residual impurities in common amounts, is modifled by adding .10 to .65% titanium thereto. That is to say, titanium is added in amounts insufiicient to combine with all of the carbon and preferably in amounts to combine with about one-half to two-thirds of the carbon or numerically stated in an amount equalling about two to less than four times the carbon content. By such additions, all of the carbon is not combined with titanium and the foregoing uncombined amount remains as iron carbide which may occur in pearlitic form or distribution. This results in the steel having enhanced room temperature properties, such as, strength, machinabilitw, etc.

At the same time, it has been demonstrated by elevated temperature tests at 1025 F. ranging up to 10,000 hours and over, that such additions are completely effective to prevent graphitization. For example, 10,000 hour tests at 1025" F. were conducted on steels of the following analyses, which may be considered as representative of steels embodying the principles of the present invention:

0 Mn 1? S Si Al M0 Ti After such tests, the steels showed absolutely no graphitization in not only the body of the steel but also in weld-heat afiected zones thereof.

In addition, it has been demonstrated that titanium increases the ultimate and tensile strength as well as the creep and rupture strength of carbon-molybdenum steels and, at the same time, decreases rather than increases the hardenability thereof.

I claim:

A method of preventing graphitization in lowcarbon, molybdenum steel containing between .05 and .20% carbon, .50 to 1.50% molybdenum and at least .02% residual aluminum comprising adding titanium to said steel in an amount between two and less than four times the carbon content to fix between one-half and two-thirds of the carbon in stable carbide form but insufiicient to prevent the formation of pearlite therein.

RUFUS E. ZIlVIMERMAN.

REFERENCES CITED FOREIGN PATENTS Country Date Austria Nov. 15, 1931 Number 

